Strain-sensitive semiconductive thin film electroacoustical transducer

ABSTRACT

A transducer for converting low-level pressure variations (such as sound waves) into electrical signal variations. A strainsensitive coating is applied to selected surface regions on a flexible diaphragm substrate. A conductive coating is also applied to the substrate flanking the strain-sensitive coating to provide electrical connections for the transducer. Sensitivity of the transducer is enhanced by placing the strain-sensitive coating at points of maximum bending on the diaphragm. The transducer may also include an integrally formed &#39;&#39;&#39;&#39;transistor&#39;&#39;&#39;&#39; to provide an active source of signal gain.

United States Patent [72] Inventor James 11. Kogen Evanston, I11. 211 App]. No. 721,309 [22] Filed Apr. 15, 1968 [45 Patented Nov. 16, 1971 [73] Assignee Shure Brothers Inc.

[54] STRAIN-SENSITIVE SEMICONDUC'IIV E THIN FILM ELECTROACOUSTICAL TRANSDUCER 7 Claims, 14 Drawing Figs. [52] U.S.Cl 179/110 B, 317/235 M [51] Int. Cl I104r 23/00 [50] Field ofSearch 179/1 10.1, 110.2; 3 10/85; 73/885 SD; 338/4; 317/235 (26) [56] References Cited UNITED STATES PATENTS 2,400,467 5/1946 Ruge 338/4 2,589,983 3/1952 Blodgett et al 310/85 3,328,649 6/1967 Rindner et a1. 179/1 10 3,351,786 11/1967 Muller et al. 317/235 Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas L. Kundert Attorney-Molinare, Allegretti, Nevitt & Wilcoff STRAIN-SENSITIVE SEMICONDUCTIVE THIN FILM ELECTROACOUSTICAL TRANSDUCER BACKGROUND OF THE INVENTION This invention relates to transducer arrangements for transforming low-level pressure changes into electrical signal variation.

In conventional microphones, a diaphragm is employed in conjunction with a transducer unit to convert acoustical pressure variations into electrical signal variations. In the so-called carbon" microphone, the sound waves deflect the diaphragm to supply pressure to carbon granules loaded within a chamber, thereby varying the resistance of an electrical path through the granules. In the crystal microphone, the motion of the diaphragm produces mechanical distortion in a crystal with consequent voltage variations being produced across the crystal due to the piezoelectric effect. In the dynamic microphone, the diaphragm is coupled to a coil. which is caused to move with respect to a stationary magnetic field, thus creating voltage variations in the coil.

While advances in the state of the art have improved the quality of these basic microphone forms such that improve ments in both sensitivity and fidelity have been realized, the use of separate but cooperating diaphragms and transducers has created a certain degree of unavoidable cost and complexity. Moreover, difficulties have been experienced in obtaining the frequency characteristics desired.

SUMMARY OF THE INVENTION It is accordingly a general object of the present invention to simply and inexpensively produce a transducer responsive to acoustical waves and pressure variations and having versatile performance characteristics adaptable for a wide range of uses.

It is a further object of the invention to transform pressure changes into electrical signal variations with the minimum of cost and complexity.

It is a still further object of the invention to simply and inexpensively produce an exceedingly compact yet highly durable microphone.

In a principal aspect, the present invention takes the form of a novel electroacoustical transducer which consists of a diaphragm upon which is deposited a thin coating of strainsensitive material. The diaphragm is mounted for bending motion in response to the impingement of acoustical or low-level pressure waves. The consequent bending motion alters the electrical properties of the strain-sensitive coating. Means electrically coupled to the strain-sensitive coating are employed for detecting the amplitude vibratory motion of the diaphragm and translating this motion into the desired fluctuating electrical signal.

In preferred embodiments of the invention, an extremely thin film of strain-sensitive material is applied only to selected portions of the diaphragm, those portions normally being the regions of the diaphragm which The strain-sensitive coating may take cadmium sulfide or the like which sputtered upon the substrate formed sistance of the thin film changing degree of strain placed upon the film as the diaphragm is distorted. In this case, the strain-sensitive film is flanked on each side by a conductive coating (which may be deposited on the diaphragm by sputtering, vacuum deposition or the like), the two conductive coatings thereby forming electrical connections to the strain-sensitive layer along a major portion of the layer's peripheral boundary.

In a further preferred arrangement, a plurality of strain-sensitive coatings are applied to the diaphragm at locations on the diaphragm having different frequency response characteristics. Accordingly, each of the different thin film regions respond to the acoustical energy impinging upon the diaphragm with diflerent frequency response characteristics, thus making possible the construction of a transducer asthe form of a thin film of is vacuum deposited or by the diaphragm, the rein direct relation to the undergo maximum bending.

sembly whose frequency response characteristics may be shaped as desired.

These and other objects, features and advantages of the present invention will be more clearly understood through a consideration of the following detailed description. In the course of this description, reference will frequently be made to the attached drawing in which:

FIG. 1 is an exploded, perspective view of a simple microphone embodying the principles of the present invention;

FIG. 2 is a side, cross-sectional view of the microphone shown in FIG. 1;

FIG. 3 is a plan view of the diaphragm illustrating the manner in which this diaphragm may be connected to an electrical sensing circuit;

FIG. 4 is a side, cross-sectional view of the diaphragm shown in FIGS. 1 through 3 illustrating the placement of the annular corrugations thereon;

FIG. 4A is an enlarged, cross-sectional view of a single corrugation on the diaphragm shown in FIG. 4;

FIG. 5 is a top view of a ribbon-type diaphragm transducer embodying the principles of the invention;

FIG. 6 is a side view of the ribbon-type diaphragm shown in FIG. 5;

FIG. 6A is an enlarged, cross-sectional view of a portion of the ribbon-type diaphragm shown in FIG. 6;

FIG. 7 is a top view of an alternative ribbon-type transducer embodying the invention;

FIG. 8 is a side view of the transducer shown in FIG. 7;

FIG. 8A is an enlarged, cross-sectional view of a portion of the transducer shown in FIG. 8;

FIG. 9 is a side view of the transducer shown in FIG. 8 with the ribbon element in flexed position;

FIG. 10 illustrates the manner in which an active transistor element may be integrally combined with the strain-sensitive coated diaphragm according to the invention; and

FIG. 11 illustrates an alternative arrangement for integrally incorporating a transistor element into the transducer.

In FIG. I of the drawings, there is shown an exceedingly simple microphone embodying the principles of the invention which comprises only three principal elements: a resonator indicated at l l; a diaphragm indicated at I2; and a rear case indicated at 13. The resonator 11 includes a plurality of sound entry apertures as indicated at 14. The rear case 13 is simply a hollow cup for defining a rear resonance volume. The resonator I1 and the rear case 13, when clamped together by retaining bolts 16 and 17 as shown in FIG. 2, firmly hold the diaphragm 12 around its entire periphery.

As illustrated in FIGS. 3 and 4 of the drawings, the diaphragm 12 includes a pair of concentric, annular depressions or corrugations molded or otherwise preformed into the diaphragm. The diaphragm may be constructed from a variety of resilient materials; however, the selected material should be electrically nonconductive or, if conductive, should be coated with an insulating layer to isolate the several conductive layers to be applied to the surface of the diaphragm. As shown in the enlarged view of FIG. 4A, one entire surface of the diaphragm may be coated with strain-sensitive material such as cadmium sulfide by known techniques such as vacuum deposition or sputtering.

The ridges of the annular corrugations constitute those regions of the diaphragm which will undergo maximum bending. Therefore, only those regions of the diaphragm need be coated with strain-sensitive material although the entire surface may be coated as indicated at 19 in FIG. 4A in order to simplify the construction process. A conductive coating is then applied to the entire surface of the diaphragm with the exception of the ridges of the corrugations 20 and 21 and the three isolating, insulating strips 27, 28, and 29- indicated in FIG. 3. The regions constituting the strips 27 through 29 and the corrugations 20 and 21 may be masked during the application of the conductive coating. The annular corrugations 20 and 21 and the strips 27 through 29 serve to separate the conductive coating into three distinct and separate regions 31, 32, and 33. The region 32 which separates the smaller annular corrugation 21 from the outer corrugation 20 has been crosshatched to point up this separate relationship, although, of course, there is no difference in actual appearance since the entire surface of the diaphragm is covered by the protective coating 34 seen in FIG. 4A.

As seen in FIG. 3, the strain-sensitive material deposited at the annular corrugation 20 separates conductive region 31 from the outer conductive region 33. Similarly, the strain-sensitive material deposited on corrugation 21 separates region 32 from the central region 31. The conductive region between strips 27 and 28 connects the central region 31 to a peripheral tab 41 which extends outwardly from the edge of the diaphragm 12. Similarly, the insulating strips 28 and 29 define a conductive strip therebetween for connecting the intermediate region 32 to the tab 42. Tab 43 is directly connected to the peripheral conductive region 33.

As will readily be understood by those skilled in the art, the impedance which would be measured between tabs 41 and 42 is the impedance of the electrical path through the strain-sensitive thin film applied to the annular corrugation 21. Similarly, the resistance appearing between tabs 42 and 43 is a measure of the resistance of the thin film applied to the outer circular corrugation 20. A source of an electrical potential 50 may be connected in series with a resistance 51 between tabs 41 and 42. In like fashion, the source 50 may also be connected through a resistance 52 to the tabs 42 and 43. The magnitude of current I flowing through resistance 51 is a direct measure of the resistance exhibited by the strain-sensitive thin film applied to the inner corrugation 21. Thus the voltage E, appearing across resistance 51 is likewise directly related to the resistance of the film at corrugation 21. In like fashion, the current I produces a voltage E across resistance 52 which is directly related to the magnitude of the resistance of the thin film applied to the outer circular corrugation 20.

The diaphragm 12 shown in FIGS. 1 through 4 flexes in response to incident acoustical or low-level pressure variations. By way of illustration, a pressure wave indicated by the arrow P in FIG. 4 tends to force the diaphragm to flex downwardly, causing strain to be applied to the thin film layer on corrugation 20. This fluctuation in strain alters the resistance of the thin film layer to produce a consequent voltage variation across the resistance 52 shown in FIG. 3. Thus, the vibratory motion of the diaphragm in response to acoustical pressure waves causes consequent electrical voltage variations across the sensing resistors 51 and 52.

Because the smaller, interior region 31 of the diaphragm 12 will tend to respond more readily to high-frequency acoustic waves than will the outer regions of the diaphragm, the variations in the voltage E will tend to have a larger magnitude at high frequencies than does the voltage E Conversely, the signal voltage E will respond more readily to low frequencies. As will be appreciated by those skilled in the art, these two voltages may be combined additively, subtractively, or in a selected phase relationship to provide a selected overall frequency response characteristic. For more specialized applications, diaphragms having different shapes and sizes may be employed with separate strain-sensitive sections connected in series or in parallel. These sections may be electrically independent as in the embodiment shown in FIG. 3, the combining if required being done in the ancillary electrical network. In addition, known frequency discriminating networks may be employed in conjunction with the plurality of transducer sections to further shape the frequency response of the overall transducer.

A further embodiment of the invention is shown in FIGS. 5, 6 and 6A. In this embodiment, the diaphragm takes the form of a preshaped ribbon wherein the strain-sensitive material is deposited as a thin film at the bending point as illustrated in the enlarged view of FIG. 6A. The ribbon itself may be constructed of glass, Mylar, Teflon, or the like, to provide an inherently insulating substrate for the strain-sensitive and conductive coatings. The strain-sensitive material 60 is applied at the bending points on a single side of the diaphragm and each is flanked by a metallic coating as indicated at 61 and 62 in FIG. 6A. As an acoustical wave impinges upon and flexes the ribbon shown in FIG. 6, each of the strain-sensitive layers is stressed in the same way. This follows because, as the angles indicated at A which form the upper V-shaped bending points tend to become more obtuse, the angles indicated at B at the bottom become more acute. Thus, with a net downward pres sure force applied, the materials applied at both the upper and lower bending points are subjected to compression. The multiplicity of strain-sensitive elements are accordingly connected in a series aiding" relationship along the length of the ribbon. The use of a conductive coating between the points of maximum bending minimizes the resistance of the transducer while maximizing its sensitivity.

An alternative ribbon-type diaphragm is illustrated in FIGS. 7, 8, 8A and 9 of the drawings. The ribbon diaphragm comprises two end sections 70 and 71 which are anchored in a support medium and a central section 73. The underside of the end of each ribbon section is bent upwardly and affixed by a suitable adhesive flush against the underside of the adjacent section, thus forming a pair of upwardly extending junctions 74 and 75. It should be noted that the ribbon diaphragm of this alternate construction may also be formed as a single integral piece. The strain-sensitive coating is then applied to the arcuate end portions 76 and 77 of the central section as illustrated in the enlarged view of FIG. 8A. As may be appreciated by a comparison of FIGS. 8 and 9 (showing the ribbon in its normal and flexed positions respectively), maximum bending takes place at these arcuate end portions.

In the enlarged view of FIG. 8A, the details of the strainsensitive structure of the ribbon transducer are shown. A piezoelectric strain-sensitive film (e.g., cadmium sulfide or cadmium selenide) may be coated on a flexible ribbon substrate 79, such as Mylar, Kapton or anodized aluminum foil. A conductive metallic film 80 is then placed over the strain-sensitive coating 78 except for the narrow strip region indicated generally at 77. A protective coating 81 (such as silicone monoxide) overlays the entire surface of the ribbon, in order to reduce the effects of humidity. This can be further improved by sealing with a thin coat of epoxy over the active area.

FIG. 10 of the drawings illustrates a third application of the principles of the invention to provide an active transducer; that is, a transistor having an integrally incorporated source of gain. A strain-sensitive coating 82 is applied at a point of maximum bending on a flexible diaphragm 83, only a portion of which is shown in FIG. 7. The strain-sensitive layer 82 is preferably composed of cadmium sulfide and is flanked on each side by metallic coatings 84 and 85 and is covered by a thin insulative layer 86 such as silicon monoxide. A still further conductive metallic coating 87 is supplied over the thin insulative layer 86. The conductive coatings 84, 85 and 87 are preferably aluminum and the diaphragm substrate 83 is made of a flexible insulating material. A conductor 91 is coupled to the metallic layer 85 while a conductor 92 is connected to layer 84. A biasing or gate" conductor 94 can be connected to the top layer 87. The path separating conductors 91 and 92 is the transconductive path for the device while a bias voltage is developed on conductor 94 which acts as the gate. The active transistor illustrated in FIG. 10 combines strain sensitivity with power gain and is basically similar in structure and function to the so-called MIPS" (metal-insulator-piezoelectric-semiconductor) transistor described in the article Transducer Action in a Metal-Insulator-Piezoelectric- Semiconductor Triode by R. S. Muller and James Conragan, V0]. V1, No. 5, Applied Physics Letters, Mar., 1965. The spacing between the aluminum layers 84 and 85 should be in the neighborhood of 25 microns; the thickness of the strain-sensitive cadmium sulfide film 81 should be approximately 3 microns; and the thickness of the silicon monoxide coating 86 separating the gate layer 87 from the strain-sensitive layer 82 should be approximately 500 angstroms. A very thin coating of epoxy can be placed over the active area to enhance the resistance of the element to the effects of humidity.

FIG. 11 of the drawings shows still another mode of incorporating gain into the transducer.

A flexible substrate 100 (of the type discussed in conjunction with FIGS. 4 and is coated with a piezoelectric film 101. A conductive layer of aluminum at 102 provides the source" for the unit while a second aluminum film layer 104 provides the drain" electrode. A third layer of aluminum 105 is maintained in spaced relation from both the piezoelectric layer 101 and the source and drain layers 102 and 104 by a silicon monoxide coating indicated generally at 110. As before, this arrangement forms a MIPS transistor. The gate layer may be left unconnected (floating") or may be connected to a biasing voltage source to provide gain adjustment capability.

it is to be understood that the embodiments of the invention which have been described are merely illustrative of applications of the principles of the invention. Numerous modifications may be made by those skilled in the art without departing from the true spirit and scope of the invention.

What is claimed is:

l. A transducer for converting acoustical pressure variations into an electrical signal which comprises, in combination,

a resilient electrically nonconductive diaphragm member mounted for bending motion in response to the impingement of low-level pressure waves, said diaphragm having at least one undulating region defined there along,

a thin coating of strain-sensitive material having piezoelectric properties deposited on said region, said material having an electrical property which varies as said diaphragm undergoes said bending motion,

a biasing conductor on said diaphragm and spaced apart from said strain-sensitive material by a thin insulator such that said transducer has an integrally incorporated source of gain, and

means for sensing the variation in said electrical property.

2. An arrangement as set forth in claim 1 wherein said strain-sensitive material is flanked on both sides by first and second electrically conductive metallic layers respectively and wherein a third metallic layer comprising said biasing conductor overlays said strain-sensitive material, said third layer being disposed in spaced apart electrically insulated relation to said strain-sensitive material and to said first and second layers.

3. A transducer for converting acoustical pressure variations into an electrical signal which comprises, in combination,

a flexible diaphragm having first and second annular concentric spaced-apart corregations defined thereon,

first and second thin coatings of a strain-sensitive material deposited over said first and second corregations respectively, said material having an electrical property which varies as said corregations are distorted due to the bending of said diaphragm in response to said pressure variation,

a first conductive coating deposited between said first and second strain-sensitive coatings and electrically contacting both first and said second strain-sensitive coatings, a second conductive coating deposited adjacent said first strain-sensitive coatings and electrically contacting said first strain-sensitive coatings and a third conductive coating deposited adjacent said second strain-sensitive coating and electrically contacting said second strain-sensitive coating, and

means for sensing the variation in said electrical property of said first and second strain-sensitive coatings.

4. A combination as set forth in claim 3 wherein the electrical impedance of said material varies in relation to the strain applied thereto.

5. A combination as set forth in claim 3 wherein the relative distortion of said first and second corrugations varies with changes in the frequency of said acoustical pressure variations.

6. A transducer for converting acoustical pressure variations into an electrical signal which comprises, in combination,

a resilient electrically nonconductive diaphragm member mounted for bending motion in response to the impingement of low-level pressure waves, having at least one undulating region defined there along,

a transistor amplifier integrally formed on said region including a source electrode, a drain electrode separated from said source electrode by a thin coating of strain-sensitive piezoelectric material having an electrical property which varies as said diaphragm undergoes said bending motion and a gate electrode spaced apart from said strain-sensitive material and said source and drain electrodes by a thin insulating material, and

means for sensing the variation in said electrical property.

7. A transducer for converting acoustical pressure variations into an electrical signal which comprises, in combination,

a flexible ribbon diaphragm member mounted for bending motion in response to the impingement of low-level pressure waves, having a plurality of undulating regions defined there along,

a plurality of coatings of a strain-sensitive material deposited over said regions on one side of said diaphragm said material having an electrical property which varies as said regions are distorted due to the bending of said diaphragm in response to said pressure variations, and

means for sensing the variation in said electrical property of said coatings. 

1. A transducer for converting acoustical pressure variations into an electrical signal which comprises, in combination, a resilient electrically nonconductive diaphragm member mounted for bending motion in response to the impingement of low-level pressure waves, said diaphragm having at least one undulating region defined there along, a thin coating of strain-sensitive material having piezoelectric properties deposited on said region, said material having an electrical property which varies as said diaphragm undergoes said bending motion, a biasing conductor on said diaphragm and spaced apart from said strain-sensitive material by a thin insulator such that said transducer has an integrally incorporated source of gain, and means for sensing the variation in said electrical property.
 2. An arRangement as set forth in claim 1 wherein said strain-sensitive material is flanked on both sides by first and second electrically conductive metallic layers respectively and wherein a third metallic layer comprising said biasing conductor overlays said strain-sensitive material, said third layer being disposed in spaced apart electrically insulated relation to said strain-sensitive material and to said first and second layers.
 3. A transducer for converting acoustical pressure variations into an electrical signal which comprises, in combination, a flexible diaphragm having first and second annular concentric spaced-apart corregations defined thereon, first and second thin coatings of a strain-sensitive material deposited over said first and second corregations respectively, said material having an electrical property which varies as said corregations are distorted due to the bending of said diaphragm in response to said pressure variation, a first conductive coating deposited between said first and second strain-sensitive coatings and electrically contacting both first and said second strain-sensitive coatings, a second conductive coating deposited adjacent said first strain-sensitive coatings and electrically contacting said first strain-sensitive coatings and a third conductive coating deposited adjacent said second strain-sensitive coating and electrically contacting said second strain-sensitive coating, and means for sensing the variation in said electrical property of said first and second strain-sensitive coatings.
 4. A combination as set forth in claim 3 wherein the electrical impedance of said material varies in relation to the strain applied thereto.
 5. A combination as set forth in claim 3 wherein the relative distortion of said first and second corrugations varies with changes in the frequency of said acoustical pressure variations.
 6. A transducer for converting acoustical pressure variations into an electrical signal which comprises, in combination, a resilient electrically nonconductive diaphragm member mounted for bending motion in response to the impingement of low-level pressure waves, having at least one undulating region defined there along, a transistor amplifier integrally formed on said region including a source electrode, a drain electrode separated from said source electrode by a thin coating of strain-sensitive piezoelectric material having an electrical property which varies as said diaphragm undergoes said bending motion and a gate electrode spaced apart from said strain-sensitive material and said source and drain electrodes by a thin insulating material, and means for sensing the variation in said electrical property.
 7. A transducer for converting acoustical pressure variations into an electrical signal which comprises, in combination, a flexible ribbon diaphragm member mounted for bending motion in response to the impingement of low-level pressure waves, having a plurality of undulating regions defined there along, a plurality of coatings of a strain-sensitive material deposited over said regions on one side of said diaphragm, said material having an electrical property which varies as said regions are distorted due to the bending of said diaphragm in response to said pressure variations, and means for sensing the variation in said electrical property of said coatings. 